This invention relates to devices for draining fluids from a tubing string in a hydrocarbon production well. Tubing drains allow fluids to drain from the tubing string of a well. Among other purposes, draining fluid from the tubing string allows the tubing to be removed from a well without pulling the tubing “wet”, which occurs when there is an obstruction in the tubing which prevents the fluid from draining out of the bottom of the tubing. For example, if the well is produced with a rod pump and the rods have parted leaving a pump or plunger at the bottom of the tubing string, the tubing will stand full of fluid unless a drain can be activated to allow the fluid to escape from the tubing into the casing-tubing annulus.
Tubing drains may be either activated by manipulation of the tubing, typically by rotation, or by applying pressure to the tubing string to a sufficiently high pressure to burst one or more rupture disks contained within the tubing drain. While each type drain has its application, the hydraulically activated drains have the advantage that rotation of the tubing is not required to activate the drain. There are situations where rotation of the tubing may not be achievable, such as in highly deviated wells or when downhole tubing or tools are stuck from casing collapse or obstructions. However, there are several disadvantages with the commonly used hydraulic drains.
One disadvantage is that if the rupture disk is unintentionally ruptured, the production equipment—usually comprising a rod string, downhole pump, and tubing string—becomes inoperable and must be removed to change out the hydraulic drain. Unintentional rupturing of the disk can, of course, be caused by the pressuring up of the tubing pressure by some event, such as the accidental closing of a valve on a surface production line. However, other phenomena may also rupture the disk. For example, the movement of rod couplings within the tubing string presents a potential mechanism for rupturing the disk. Physical contact between the rod coupling and the disk can cause rupturing of the disk by the impact by the coupling upon the disk. In addition, the motion of the coupling in close proximity to the hydraulic drain can cause a localized pressure spike resulting from the piston effect of the coupling inside or adjacent to the drain. The likelihood of such premature rupturing of the disk increases with the decrease in clearance between the rod coupling and the inside diameter of the hydraulic drain.
Another disadvantage of hydraulic drains is that many of the drains utilize elastomeric O-ring seals which can degrade over time, particularly in the presence of corrosive wellbore fluid, harsh downhole treatment fluids, high temperatures, and/or high pressures. A seal failure will result in fluid leakage from the tubing which requires the removal of the tubing string to change out the drain.
Another disadvantage of some hydraulic drains is that the rupture disks are unrestrained such that the disk remnants end up inside the well, leaving junk/trash which can either interfere with the operation of downhole equipment or which can accumulate with other debris to create a wellbore obstruction.
Another disadvantage of the known hydraulic drains is that the replacement of a rupture disk within the hydraulic drain typically requires sending the drain into a shop for replacement of the rupture disk and related elastomeric O-ring seals. If the hydraulic drain is of the type which utilizes threads in the mandrel for retaining the rupture disk, the threads may be damaged and require redressing. The life of the drain may be limited if the threads are damaged through repeated use because satisfactory repair of the threads may not be possible, which means the mandrel can no longer be used.